The invention relates to switch assemblies, and in particular relates to a reliable contact block with a double break spanner.
Electrical switches, such as pushbuttons or rotary switches, and the like, used for the control of industrial equipment, are typically mounted onto a front panel of a cabinet so that the manipulated portion of the switch (termed the “pushbutton operator”) projects out from and is accessible at the front of the cabinet.
For a pushbutton switch, a hole of sufficient diameter may be punched in the cabinet to accommodate the threaded portion of the operator The threaded portion is inserted through the hole, and secured to the panel with a threaded retaining nut. The panel is thus sandwiched between the operator and the retaining nut.
A latch assembly is mounted on the end of the operator protruding inside the panel and a contact block or a plurality of contact blocks are mounted onto the other side of the latch assembly. The contact blocks are electrically connected to the circuit or circuits that the switch is to control.
Contact blocks typically include housings that contain normally open and/or normally closed contacts. A normally open contact may be used, for example, when a user wishes to activate a specified function by actuating the operator, thereby closing the normally open contact. When the operator switch is deactivated, a plunger returns to its normal position, thereby opening the normally open contact and terminating the controlled function.
A normally closed contact may be used when a user wishes to stop an ongoing function. One common example of a normally closed contact is an Emergency Stop (E-Stop) function which is activated when the user wishes to immediately terminate the controlled function due, e.g., to a malfunction in the process or the development of a situation that may cause damage to the product line or the operating equipment. In this situation, when the switch operator is actuated, the normally closed contact opens and remains open until the operator is returned to its normal state, thereby closing the normally closed contact and resuming the controlled function.
Referring to FIG. 1, a conventional switch 20 is illustrated including a spanner 21 that is disposed above a pair of contact plates 24. Spanner 21 is a double break spanner, meaning that both outer ends 22 engage a contact plate 24 such that the circuit is broken if either outer end becomes disengaged from the corresponding contact plate. In particular, each contact plate is aligned with an outer end 22 of spanner 21. Spanner 21 and contact plates 24 are of the type that are installed into a contact block (not shown) in the general orientation illustrated. A switch operator of a pushbutton, for instance, may be depressed (in a normally open switch) to bias spanner 21 downwardly along the direction of Arrow A until the outer ends 22 engage the corresponding contact plates 24 to operate a controlled function. A contact 26 is in the form of a conductive nub that protrudes upwardly from each contact plate 24 and towards a corresponding outer end 22 to provide a contact location between the spanner 21 and contact plates 24 when the switch 20 is closed. The pushbutton is released to allow the spanner 21 to translate upwardly away from contact plates 24 under a spring force along the direction of Arrow B to disengage the outer ends 22 from the contact plates 24 when operation of the controlled function is to be discontinued. It has been recognized, however, that the accumulation of a nonconductive mass (such as dirt, dust and the like) may become lodged between the contact 26 and outer ends of spanner 22, which prevents current from flowing through the closed switch 20. Electrical conduction between contact plates 24 was thus not reliably established in conventional switch 20.
Referring now to FIG. 2, a conventional switch 28 addresses the potential presence of nonconductive masses that could compromise the reliability of switch 20 illustrated in FIG. 1. In particular, switch 28 includes a spanner 30 having a slot 32 extending longitudinally partially through each outer end 34 to produce a pair of bifurcated fingers 36a and 36b at each end. Each finger 36 is independently vertically flexible with respect to the spanner 30 and therefore provides a redundant contact that engages a flat contact plate 38. A contact (not shown) protrudes downwardly from the lower surface of each finger 36 towards the contact plate 38. Accordingly, if a nonconductive mass were to become lodged between one of the contacts (e.g., of a finger 36a) and contact plate 38 to prevent the corresponding finger 36a from making electrical contact with the plate, the contact corresponding to the adjacent finger 36b would still engage the contact plate 38 to enable current to flow through spanner 30. Unfortunately, when switch 28 is opened, an electrical arc is often created between the contact plate 38 and the last finger 36 to disconnect from the plate 38. Because the bifurcated fingers 36 have a reduced mass with respect to the outer end 34, the fingers tend to melt or otherwise fail in response to the heat produced by the arc.
What is therefore needed is a switch usable in a contact block that provides redundancy without compromising the structural integrity of the switch components during use.